Should you seal damp walls before plastering?

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should you seal damp walls before you plaster

When a property has suffered with damp, there are usually additional repairs that need carrying out inside your home. This is because damp can cause serious damage to internal decorations.

One area that often suffers is the internal plaster of a property. Standard plaster is quite porous, and water easily soaks in. This often includes hygroscopic salts, which can add to damage on the internal walls of your property.

Some of the common signs of damp internal plaster include:

  • Visible damp patches. These can be isolated patches in the case of penetrating damp, or tide marks lower to the floor if you are suffering with rising damp.

  • Mould and mildew. This is a clear sign that damp is present. because moulds need moisture to grow and spread.

    Ideally any type of mould needs removing ASAP, as the spores released into the air can be damaging to your health. This is particularly true for the elderly and people with respiratory conditions

  • Hygroscopic salt deposits. When water is drawn through the masonry of your property, it also brings natural salts. This is particularly true for rising damp that originates from the ground beneath your property.

    Common signs of salts include, crumbling and cracking plaster, fluffy looking white growths on damp patches, plus blistering and peeling of paints and wallpaper.

When you have identified any of these issues, you must then discover where the damp is coming from. Once you know the cause of the problem, you should carry out repairs to stop the damp issue at its source.

You should completely stop the source of damp and seal the wall before you decide to replaster. If you fail to do this correctly, then the original damp issue is likely to persist.

How to identify what is causing your damp issue

As we have already mentioned sealing damp walls before plastering is extremely important. If you skip this step and simply replaster without addressing the root cause, it will not be long before the issue returns in your new plaster.

There are three main types of damp:

Will you need to replaster due to condensation damp?

Of the three types of damp, condensation is the most common. In fact, most homes in the UK will suffer with some level of condensation.

This is because people produce a lot of moisture. It comes from a variety of sources, such as bodily functions, like breathing and sweating. As well as household tasks, like bathing, cooking, and drying clothes.

The good news is condensation damp is usually a surface issue. Unless the level of condensation is significant and has been left for a long period of time, it is unlikely that it will have soaked deep into plaster.

If this is the case, simply improving ventilation and airflow may be enough to solve this issue. You may also need to remove any mould and mildew and redecorate. But it is unlikely that the plaster will need replacing.

Sealing damp walls and replastering after penetrating damp

The second most common type of damp is penetrating damp. The reason this type of damp is so common, is it can affect any area of your property.

Penetrating damp describes water ingress that occurs due to external defects in the property. This could include any of the following:

  • Damage to external renders
  • Old worn and damaged pointing
  • Cracked brickwork
  • Failed sealant around windows and doors  
  • Defects in your roof or chimney stack
  • Debris and rubble bridging your cavity wall

Penetrating damp can be identified as isolated damp patches on internal walls. They are usually localised to the area adjacent to the external defect. This means you can usually see the damp inside, then locate the issue outside with a little investigation.

The only situation where this is not true, is if bridging is causing the issue. This is where moisture passes across the cavity walls of a property.

In this case, water could travel down the cavity and then bridge the gap to your internal wall. This usually happens when debris lodges in the cavity, forming a bridge from one side to the other.

If bridging is an issue, locating the defect can be more difficult. The reason for this, is the defect could be in a completely different location to the resulting internal damp.

If you are struggling to locate a defect, it is highly advised that you have a professional damp survey. This could save you a huge amount of money in the long run.

For damp surveys we recommend Bark, they can get you up to 5 quotes from local damp surveyors. This means the survey costs are usually very affordable.

Click here to get up to 5 damp survey quotes online now

Whatever your problem turns out to be. You need to fix the external defect to seal the wall if you intend to replaster.

You should also allow a period for the wall to dry out. If high levels of moisture remain in the masonry, the damp patches could return, even after the defect is completely fixed.

Sealing damp walls and replastering after rising damp

The last type of damp is rising damp. As the name suggests, this is damp that is rising from the ground.

Thankfully, this type of damp is the least common. However, when it does happen, it often brings the worst issues. This is particularly true for your internal decoration and plaster.

Rising damp will often completely saturate masonry and plasterwork. It also contains high amounts of hygroscopic salts. This means that the damage it causes can be more serious and unsightly.

As with penetrating damp, you will need to resolve the issue before you can start to add new plaster and redecorate. This will require you to stop the damp rising, by repairing your existing damp proof course, or adding an entirely new one.

The easiest and most popular way of doing this, is with an injected chemical damp proof course. This is a cream that is injected into the mortar of your property, just above ground level. It creates a waterproof barrier that completely stops rising damp.

Following this, you will need to allow the affected area to completely dry before you begin to replaster.

Extra steps to seal your damp walls before replastering

Once you have completely removed the causes of damp and sealed the walls from excess moisture, you are free to replaster. At this point, you can just apply a standard plaster to your walls to finish the job and then redecorate.

However, in some cases you may want to take extra steps, to ensure the damp patches do not return. This is especially true if there was a significant amount of hygroscopic salts present.

These salts can remain in the wall and hold moisture. It is not uncommon for this to penetrate through later, even after repairs have been completely successful.

Below are a few additional steps you can take internally:

  • Use a tanking slurry. This creates a completely waterproof barrier between your wall and the finished plaster. Simply apply tanking slurry to the affected wall and then plaster over the tanking slurry. You can read more about tanking slurry by clicking here

  • Apply a damp proof plaster. There are several specialist damp plasters on the market. These offer a waterproof finish that is salt resistant and breathable. This can be finished with a standard skim coat.

  • Once your plaster has been applied you can use an internal damp proof paint. This will add an additional layer of protection after the wall is sealed and plastered. You can read more about internal damp proof paints here


So, should you seal damp walls before plastering? In this article I think we have shown that the answer to this is yes. Sealing damp walls before plastering is essential if you do not want the problem to return.

As with most things in life, you do not want to simply gloss over a problem. You need to tackle it at its source. When you put in this small amount of extra effort, you will be rewarded with a finished job, that not only solves your problem, but will also stand the test of time.